Add validation and hints to delete blob model

This is a follow-up to MR !65293 (merged), we need to add validation and hints to the "Delete Blob Modal".

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Please reference > https://gitlab.com/gitlab-org/gitlab/blob/master/app/views/shared/_commit_message_container.html.haml

• Try to keep the first line under 52 characters and the others under 72.
• Include description in commit message

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